Many integrated circuit devices include an oscillator which provides a periodic output signal for normally controlling operation of the device. In some integrated circuit memory devices, the oscillator plays a role in which the frequency of the periodic output signal is not critical, provided that it remains broadly within predefined parameters. However, in integrated circuit flash memory devices, the oscillator plays a more critical role to control the programming and erase times of flash memory cells and also to control the other timing events relating to programming and transitions of states between different modes. Thus, in an integrated circuit flash memory device it is particularly important to be able to monitor and control the frequency of the periodic output signal of the oscillator. It will readily be appreciated that this frequency can vary for a plurality of different reasons, including in particular variations in process technology and the operating temperature of the integrated circuit device.
Conventional memory testers are not equipped to measure the frequency of a free running periodic signal. It will be appreciated that reference herein to an integrated circuit device generally implies a single chip device or a plurality of chips in a single package. To measure the frequency of a periodic output signal from an oscillator on chip, existing technology requires that the periodic signal be transmitted off chip to an external piece of equipment which includes for example an interval counter. An alternative method of measurement using existing technology is to provide a phase locked loop and frequency to voltage converter at the test interface with the chip to convert the frequency of the periodic output signal of the oscillator into a voltage which can then be measured by a conventional memory tester. In these situations, the integrity of the periodic signal must be guaranteed up to the point at which the measurement is taken. It will readily be appreciated that for a periodic signal, particularly one at a high frequency, this is not always easy to accomplish. Moreover, it will readily be understood that with existing technology specialized pieces of equipment in addition to a conventional memory tester are required to measure the frequency of the output signal of the oscillator.
In GB-A-2217465 (Sun Microsystems Inc.), a terminal count detector is used. Use is made of a count generated by the tester to be applied to the part. When the part detects that the count has reached a predefined limit fixed by design, it disables the oscillator and a reading of the clock input counter may be made from which a frequency is derived. Using the tester to produce a count in devices where the pin count is low only allows a small number of bits to be assigned to this test mode so reducing the resolution to which an interval time can be controlled.
IN US-A-5099196 (Long et al.), a comparator is used in a similar way to the terminal count detector of GB-A-2217465. Latches produced on the chip are loaded with what would represent the test limit of the frequency. The oscillator is then enabled and disabled, but the result of the counter is not available to be brought out to the tester. Instead, a comparator exists on chip which compares the value stored in the latches and the value of the oscillator counter. The result of this comparison produces a go-no go test for the frequency. Thus, this document does not give the value of the frequency in one test but instead a search has to be conducted to find the frequency. Furthermore, it adds extra complications to the circuitry which costs in terms of silicon area.
Thus, both of these prior art proposals involve adding extra devices to the chip to provide the necessary functionality for testing.
It is an object of the present invention to provide a simpler and more reliable way of measuring the frequency of a periodic signal output from an on-chip oscillator in an integrated circuit device.